Geology

Geologic History

The area now known as Loudoun County has had a long and complex geologic history. The oldest rocks were folded and thrust up into mountains over one-billion years ago, eroded to flat plains, and lifted up into mountains again. Younger rocks were made from sediments deposited in ancient seas and then turned into rock under heat and pressure. Later, molten rock from deep within the earth forced its way up toward the surface through these older rocks before slowly cooling. These geologic processes, some of which continue today, shaped the land that we now live on.

Physiographic Provinces

The eastern half of Loudoun County is located in the Piedmont physiographic province and the western half in the Blue Ridge physiographic province. The Bull Run fault, coincident with the eastern edge of the Catoctin Mountain, forms the boundary line between the two provinces. Topography in Loudoun varies with elevations ranging from 180 feet above sea level in the east (at the Potomac River) to 1,900 feet above sea level in the west (along the crest of the Blue Ridge).

Major drainage systems include Broad Run, Goose Creek, Catoctin Creek, and Bull Run. These drainage systems and all of the smaller ones in Loudoun County are part of the Potomac River Basin.

Groundwater in Loudoun County

The geologic characteristics of bedrock and unconsolidated sediments affect the occurrence of groundwater and its availability to wells. Groundwater is the sole source of water for domestic use for many Loudoun residents and it also seeps into stream channels to provide virtually all flow during dry periods. Read more about groundwater in Loudoun County.

Learn More

Special geologic and soil conditions may result in potential problems for some types of land use. County staff having expertise in geology, soil science, geotechnical engineering, and/or hydrogeology review development proposals in these areas to help minimize environmental impacts and assure public health and safety. To learn more, please contact us at 703-777-0220.